The present invention relates to an apparatus for processing radiated digitised radio frequency (RF) signals, and relates particularly, but not exclusively, to an apparatus for processing radiated digitised frequency modulated (FM) radio frequency signals.
Radio frequency transmission of digital signals is usually achieved by means of frequency-shift keying (FSK), in which a radio frequency (RF) carrier signal is shifted in frequency between two closely spaced frequencies, respectively above and below the carrier frequency, representing the 1""s and 0""s of digital serial bits transmitted. A received FSK signal is detected by processing the carrier signal by means of an analog receiver having a local oscillator to produce a signal at an intermediate frequency which is a frequency shifted version of the RF signal at the carrier frequency. The signal at the intermediate frequency is subsequently fed to a limiting amplifier and then to a detector, which provides an output signal having an amplitude representing the digital frequency modulating signal. The output from the receiver is in the form of a series of data bits, and is then processed by means of a digital circuit which either provides output analog signals representing the original analog signal of which a digital version is transmitted, or output signals to enable various functions to be carried out, such as automatic telephone dialling by security apparatus, in response to the RF signal being transmitted.
Existing apparatus for processing digitised RF signals suffers from the drawback that interference between the local oscillator of the analog receiver and a clock generator, which is needed to operate the digital processor limits the extent to which the analog receiver and digital processor can operate in proximity to each other because of problems such as noise generation and the like. In order to minimise such interference between the local oscillator and clock generator, these two components need to be shielded from each other by means of cumbersome and bulky shielding arrangements, which increase the cost of apparatus and limit the extent to which the size of the apparatus can be reduced.
In existing apparatus, each track in the circuit will to some extent carry harmonics of the processor clock frequency. Any control lines driven by the processor will also occur on a xe2x80x9cclock edgexe2x80x9d and therefore any action performed by the processor will generate radiated and directly injected noise.
Existing apparatus therefore has to include components for filtering out such radiated and directly injected noise and also to buffer such noise. Such components also add to the cost of the apparatus and limit the extent to which the size of the apparatus can be reduced.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.
According to the present invention, there is provided an apparatus for processing radiated digitised radio frequency signals, the apparatus comprising an analog receiver having at least one local oscillator, wherein the analog receiver in use receives the first electrical signal from an antenna at a carrier frequency, is modulated by a digitised modulating signal and provides a second electrical signal having an amplitude representing an instantaneous logic level of the digitised modulating signal; and
a digital processor for processing the second electrical signal and providing an output signal in response thereto, wherein the digital processor in use is controlled by clock signals provided in response to actuation of a local oscillator of the analog receiver.
By providing clock signals in response to actuation of a local oscillator of the analog receiver, the clock frequency can be arranged to be an integral fraction of the local oscillator frequency. Both the clock signals and the signals from the local oscillator may be viewed as originating from a single source as they share a common control source. This means that the noise generated by the processor is in harmony with the second local oscillator and therefore the effects of the noise generation are substantially eliminated. This provides the advantage of minimising interference between the local oscillator of the analog and the clock generator of the digital processor, which gives rise to the further advantage of improving the extent to which the analog receiver and digital processor can be located close to each other in a single device.
The output signal of the digital processor may be an analog version of the digitised modulating signal. In particular, the output signal may be an audio signal.
Alternatively, or in addition, the apparatus may be security apparatus wherein the output signal of the digital processor effects one or more tasks in response to receipt of a radiated RF signal.
The radiated radio frequency signals may be frequency modulated signals.